Automatic control system for antilocking and antiskid applications

ABSTRACT

Instantaneous coefficient of friction μ(K) and slope K.sub.μ  of the μ-slip curve are evaluated from the drive torque or the brake torque and the wheel speeds. The values of K.sub.μ  obtained by successive calculations constitute the feedback of the control system, which is compared with a target value K.sub.μtar. The connection value thus obtained influences a final control element which modifies the braking pressure or, for example, the quantity of fuel per unit time. The control system gain varies with the adhesion coefficient μ(K).

BACKGROUND OF THE INVENTION

From DE-OS 35 35 843, for example, it is known to estimate parameters byusing the measured speed of a wheel and the measured brake pressure insuccessive sensing times . . . (T-1), T, (T-1) . . . by means of adifference equation and by using known identification algorithms. Theseparameters are used to determine estimated values μ.sub.(t) for thefriction coefficient between tire and road. The μ-values serve todetermine the slope of the slippage curve ##EQU1## in the operatingpoint and can be used in an anti lock brake control to achieve anoptimal control. The slope K.sub.μ of the μ-curve serves as a controlvalue, and the brake pressure is controlled such that K.sub.μcorresponds to a small, positive value. The slippage selected is alwaysone that is to the left of the maximum (on the stable portion) of theμ-slippage curve, however, very close to the maximum.

SUMMARY OF THE INVENTION

The invention is based on the known control system and broadens it toinclude the ASR case and improves the system in that it makes the gainadditionally dependent upon the determined μ and correspondinglycalculates the control amplifier.

In the ABS case, the continuous manipulated variable (e.g. theproportional valve) influences the brake pressure In the ASR case, inorder to change the drive torque, the amount of fuel supplied per timeunit, the ignition or the brake pressure at the driven wheels independency upon K.sub.μ are varied. In both cases, the wheel speed ismeasured and, further, a value corresponding to the brake torque and/ordrive torque is determined (e.g. the brake pressure and/or the amount offuel injected per time unit).

Since the manipulated variable is continuous, the control is hence alsocontinuous.

Moreover, the control is adaptive since K.sub.μ is not measured directlybut determined by parameter estimation. Further, the control parametersare constantly adjusted to the instantaneous μ-values.

The control of the invention makes optimal or nearly optimal use of thefrictional connection. Since there are no gear change thresholds, thiscontrol is user-oriented.

BRIEF DESCRIPTION OF THE DRAWING

The single figure shows a control circuit in an ABS application.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

In the figure, the vehicle-tire-road-system bears reference numeral 1. Abrake pressure P_(B) is supplied to this system. The wheel speed V_(R)is indicated as an output value on the right side. Signals correspondingto the values P_(B) and V_(R) are supplied to an estimator 2. The lattersupplies at successive sensing times K estimated values μ.sub.(t) andT.sub.μ the instantaneous friction coefficient and/or the instantaneousslope of the μ-slippage curve. This is carried out in a manner knownfrom DE-OS 3535843 or in any other known way or in ways still to befound. The estimated value K.sub.μ is supplied to the subtractor 7 as anactual value. Also supplied thereto is a small positive value K.sub.μtaras a target value. Then, the difference of the actual value signals andthe target value K.sub.μtar is, as a control value, supplied to acontrol amplifier 3. Preferably, the control amplifier 3 exhibitsproportional and integral behavior. As will be shown later, it will beadjusted to the instantaneous μ.sub.(t). The output signal P_(H) thereofis filtered in a filter 4. The so resulting value P_(M) influences acontinuously adjustable final control element, e.g. a proportioningvalve, via which the necessary brake pressure P_(B) is supplied to thewheel brake (in I).

In order to achieve an improved dynamic behavior and an improvedstability of the control loop, an adjusting element 6 is used tocontinuously adjust the gain factor K_(R) of the control amplifier 3 independency upon the determined μ.sub.(t). This is preferably doneaccording to the following rule:

Initial setting: K_(R) =K_(R),O (for the best but still stableselection, e.g. for μ=0.1) ##EQU2##

Based on the initial setting K_(R),O of the gain factor, an adjustmentfactor G_(F) is calculated in each calculation step (=sensing intervalT_(A)). This adjustment factor G_(F) then determines the new selectionK_(R). Further, it is dependent upon μ, with σ being the adjustingspeed. σ can again be selected as constant or in dependency uponμ(μ=large, then σ=large).

The control circuit for an ASR-case differs from the one of the singlefigure in that the input value of the vehicle-tire-road system is notthe brake pressure but, for example, the amount of fuel M_(E) suppliedto the engine per time unit. This amount is, for example, measured andsupplied to the estimator 2 which determines μ.sub.(t) and K.sub.μ. Heretoo, the actual value is K.sub.μ and the target value is againK.sub.μ,tar ≈0 or equal to 0. The signals at the outputs of the elements3 and 4 do not correspond to pressures but to amounts of fuel per timeunit. Manipulated values for this purpose are, for example, known inconnection with E-gas.

I claim:
 1. System for controlling torque at a vehicle wheel moving at aspeed, said torque being one of driving torque or braking torque, saidwheel upon application of torque exhibiting slippage characterized by aμ-slippage curve having a stable portion, an unstable portion, and amaximum therebetween, said system comprisingmeans for measuring thespeed of said vehicle wheel and producing a wheel speed signal, meansfor determining the torque applied to said vehicle wheel and producing awheel torque signal, means for estimating an instantaneous frictioncoefficient μ(t) and a corresponding instantaneous slope K.sub.μ of theμ-slippage curve from said wheel speed signal and said wheel torquesignal at respective sensing times (T-1), T, (T+1) . . . , means fordetermining a difference between K.sub.μ and a target slope K.sub.μtaron the stable portion of said μ-slippage curve. a control amplifierwhich generates an output signal from said difference, said controlamplifier having a gain factor K_(R) which changes in dependence uponthe instantaneous friction coefficient μ(t), and a final control elementwhich generates said wheel torque signal from said output signal. 2.System as in claim 1 wherein said gain factor is changed in proportionto the instantaneous friction coefficient μ(t).
 3. System as in claim 2wherein said gain factor is changed according to the equation K_(R)=K_(RO) /(1-σμ), where K_(RO) is an initial setting and σ is anadjustment speed.
 4. System as i claim 3 further comprising adjustmentmeans which limits an adjustment factor G_(F) =1-σμ by minimum andmaximum values.
 5. Method for controlling torque at a vehicle wheelmoving at a speed, said torque being one of driving torque or brakingtorque, said wheel upon application of torque exhibiting slippagecharacterized by a μ-slippage curve having a stable portion, an unstableportion, and a maximum therebetween, said method comprisingmeasuring thespeed of said vehicle wheel and producing a wheel speed signal,determining the torque applied to said vehicle wheel and producing awheel torque signal, estimating an instantaneous friction coefficientμ(t) and a corresponding instantaneous slope K.sub.μ of the μ-slippagecurve from said wheel speed signal and said wheel torque signal atrespective sensing times (T-1), T, (T+1) . . . , determining adifference between K.sub.μ and a target slope Kμtar on the stableportion of said μ-slippage curve, generating an output signal from saiddifference using a gain factor K_(R) which changes in dependence uponthe instantaneous friction coefficient μ(t), and generating said wheeltorque signal from said output signal.
 6. Method as in claim 5 whereinsaid gain factor K_(R) changes in proportion to the instantaneousfriction coefficient μ(t).
 7. Method as in claim 6 wherein said gainfactor changes according to the equation K_(R) =K_(RO) /(1-σμ), whereK_(RO) is an initial setting and σ is an adjustment speed.
 8. Method asin claim 7 wherein an adjustment factor G_(F) =1-σμ is limited byminimum and maximum values.